Method and system for electron irradiation of materials



March 30, 1965 3,176,129

mawnon AND SYSTEM FOR ELECTRON xmmnn'rxon OF MATERIALS E. J. LAWTON ETI'AL Filed Oct. 23. 1961 Bias and Wlage Signal Source d Dept]! T im:

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In venfars h, c $2M 06 A MFW J8 h. #M T EA Dept/l United States Patent 3,176,129 METHOD AND SYSTEM FOR ELECTRON IRRADIATION OF MATERIALS Elliott J. Lawton, Schenectady, N.Y., and Anatole M.

Gurewitsch, Zurich, Switzerland, assignors to General Electric Company, a corporation of New York Filed Oct. 23, 1961, Ser. No. 146,705 8 Claims. (Cl. 250-495) The present invention relates to a system and method for irradiating materials with electrons.

When some materials are subjected to a beam of high energy electrons, the resulting ionization energy absorbed in the materials may produce beneficial results such as sterilization and de-infestation of foods and drugs, acceleration of chemical reactions, coloration of glass, treat ment of skin diseases, etc. Usually, best results are obtained when the ionization is substantially uniform throughout the thickness of material being irradiated.

Accordingly, an object of the present invention is to provide a method for producing more uniform ionization by electrons throughout the thicknesses of irradiated materials.

Another object is to provide an electron beam system for producing more uniform ionization distributions throughout the thicknesses of irradiated materials.

An electron beam system provides more uniform ionization distribution when the electron beam is made up of a range of energies, from low to high, such as can be realized with an accelerating voltage which varies with time. On the other hand, when varying accelerating voltages are used, the low magnitude portions of these voltages result in a greater energy loss in the vacuum window provided at the beam exit area of the system. The window may absorb so much energy that it is destroyed by the resulting heat. Consequently, electron beam systems are usually operated with nearly constant high accelerating voltages to avoid overheating of the window even though better ionization distribution would be obtained throughout the thicknesses of the material with a varying accelerating voltage.

Therefore, an object of the present invention is to provide an improved electron beam system in which the beam accelerating voltage varies with time but in which the Window heating is not destructive. I

A still further object of the present invention is to provide a method of operating an electron beam system to obtain more uniform ionization distribution without destructive window heating.

These and other objects are achieved in a preferred embodiment of our invention in which the electron beam current magnitude in an electron beam generator, operated under space charge limited conditions, is varied in phase with the beam voltage to produce substantially uniform ionization distribution.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description, taken in connection with the accompanying drawing in which:

FIG. 1 is a partially sectioned side view of an electron beam generator with which the present invention can be practiced,

FIG. 2 is a schematic diagram of the electron gun in the electron beam generator of FIG. 1,

FIG. 3 comprises graphs illustrating the ionization distribution produced by conventional operation of an elec tron beam generator,

FIG. 4 comprises graphs A, B and C illustrating electrical waveforms of operation when the electron beam generator of FIG. 1 is operated in a preferred manner,

FIG. 5 is a graph illustrating the synthesis of an ionization distribution resulting from a sine wave electron beam whose current is in phase with a sine wave acceleration voltage, and

FIG. 6 is a block diagram of one possible system for producing a control voltage for the grid electrode of the electron gun of FIG. 2.

Referring now to FIG. 1, we have illustrated an electron beam generator that, except for the electron gun, may be conventional. In this generator a tube 1 contains components (not shown) for producing and accelerating an electron beam 2 that then passes through a neck 3 of tube 1 having a terminating flared portion 4. By way of example, at the bottom of portion 4 a long narrow slot is covered by a vacuum-tight thin metallic foil 5, called a window, that is bowed due to the pressure difference caused by the high vacuum inside portion 4 and the atmospheric pressure outside. Other shapes of windows could be used. The high energy electrons in beam 2 readily pass through thin window 5 to irradiate a material 6 which may be moved by a belt 7 normal to the long dimension of the flared portion 4. Beam 2, which may be deflected parallel to the long dimension of portion 4, as well as transversely, irradiates substantially the total volume of material 6.

In FIG. 2 we have illustrated the electron gun for the accelerating tube 1 of the electron beam generator in FIG. 1. In this electron gun a filament 8 emits electrons that pass through a grid electrode 9 under the magnitude control of a voltage, which we shall identify as V applied across two input terminals 10 from a source 11. These electrons are then focused by voltages applied to a focusing electrode 12 and a high voltage electrode 13; the voltage on the latter also acts with voltages on other electrodes 14, 15, etc., as a means for accelerating beam 2. The cathode and grid structure may be such as set forth more fully and claimed in Patent No. 2,914,692, granted November 24, 1959, which patent is assigned to the as signee of the present application. The electron gun elements and electrodes can be energized by a resonant transformer 16 such as the type described and claimed in Patent No. 2,144,518, granted January 17, 1939, which patent is assigned to the assignee of the present application.

In some high energy electron beam generators, the electron gun is operated under emission limited conditions. In other words, the magnitude of current in the electron beam is usually all of the current that the filament electrode in the electron gun is capable of producing at'the temperature at which it is operating. There is little control of current since all of the current emitted from the filament is used in the electron beam. In the present apparatus, grid electrode 9 is biased to provide space charge limited operation. In this type of operation a cloud of electrons is formed a short distance from filament 8, from which the number of electrons forming electron beam 2 is determined by a varying voltage V applied to the grid electrode 9. The shape of this varying voltage is significant, as is explained below.

Referring now to FIG. 3, an ionization distribution graph 17 is illustrated fora relatively constant potential electron beam generator. The ordinate values correspond to relative ionization and the abcissa values to depth in the irradiated material 6. Graph 17 illustrates that when a material is irradiated with high energy electrons of substantially constant potential, the distribution of the ionization throughout the thickness of material varies. The ionization is low near the surface and then increases to a maximum at a depth d For depths greater than d the ionization gradually decreases to zero. As perviously mentioned, uniform ionization distribution is desired. Thus, the ionization distribution corresponding to curve 17 is far from ideal. However, the uniformity of ionizalarger In FIG.

tion beyond the depth d canbe improved by irradiating the material 6 from both sides. Graph 18 corresponds to the irradiation from theother side. -As a result of this second irradiation the ionization distribution'between V depths d and do is more uniform. .Thatiis, the total ioni-' zation, as illustrated by curve 19, which can be found by adding the ordinate values of curves 17 and 18, is relatively constant within the interior of material 6, whose thickness is indicated by T, inFIG. 3. ,7

Even though the ionization can be made more uniform near the interior by the irradiation from both siles of material 6, there'still remains thelarge variation in ionization distribution near the two irradiated surfaces. This variation could be largely eliminated by the use of.

" absorbers placed on the surfaces of material 6 or between 1 the material 6 and the tube window. These'absorbers, in efiect, present a thickness to the irradiation, and thus the large Variation in ionization distribution between the surface and depth d occursin the absorbers rather than in V material 6 This, of cou rse,'is a very costly procedure as the'fracti on of the total energy of the electron beam that must be dissipated in the absorber is completely wasted near the surface is obtained without the use of absorbers,

' and also without significant window heating. The electrical characteristics of operation that provides this result are illustrated in the graphs ofFIGS. '4A, 4B, and 4C.

In FIG. 4A there is illustrateda half cycle, .21, of. the j aceelerationvoltage V for an electron. beam generator in which the accelerating potential varies with time, for I example, sinusoidally. V is the positive voltage on wina dow 5 as Compared to filament 8. 'Actually, window 5.

,is usually grounded and filament 8 maintainedat a large a negative potential bel'ow' ground. Voltage 1V; is the half.

of eachcycle during'whichfelectrons leave filaments. It" 1 contains all values of voltage from substantially zero to the maximum value; t t

InEI G. '4B there isillus'trated a graph- 22 of a preferred voltageV that is applied to the grid electrode 9 with respect to the filament 8Q This voltage V is negative. With" space charge limited operation, this :voltage V causes theamplitude' of the beam current I represented by a curve'23 inFIG. 4C, to vary in phase with the acceleration voltage V such thatthere is'lo w'b rent for low bearn'voltage.

; "The significance of this shape vofthe beam current I5 relates to the proportion of the'beam energy dissipated in window 5, whichis greater for low than for high beam.

Voltages. Consequently, by decreasing the beam current for low beam voltages the amount of energy lost in window S isdecreased and overheating of window 5 is readily V obviated. The beam current I should notjbe' decreased 1 .tozero at low beam voltages sincethefresultingbeam energies are very important in producing ionization near the surface of material 6 Where, as is indicatediby curve' 17in FIG. 3, there is insufficientionization'between'the V surface and a depth d The ionization near the surface can be increased by allowingcurrent to flow during the f Same hs in the material a low voltage portions'of the accelerating voltage V,,. However, this; current must be icompatible with permissible window heating. This currentisobtained when'thebeam current l is .inphase with the beam voltage V and has a shape similar to that of the voltage V,,. Then, when I voltage V is at low magnitudes, at which there is danger 0t window overheating, the beam current I is atla low t magnitude thereby avoiding window overheating. "At higher voltages where there is less danger of window overheating, the beam current 1 is "correspondingly U i i with respectto ground} 45 '5 have illustrated the synthesis or the r nt zation curve 2 fora sine wave of beam current in phase with a sine wave of beam'voltage of 3 megavolts maximum amplitude such as would be. obtainedusing an inphase sine Wave biasing voltage, curve 22of FIG. 4. The resultant curve at is obtained by adding the ordinates of curves 24, 25, 26' and 27, which representtheinstantaneous ionization distributions occurring when the acceleration' voltage sine Wave has reached amplitudes of 53.2, 1 and /2 megavolts, respectively, Actually curve 2% is takento represent thecurve obtained by summing the infinite number of ionizationdistributions occurring during each half cycle.

A different resultant distribution curve can be obtained by altering the relative beam current variation by controlling the sliape of the bias voltage, V which, in turn, changes the'magnitude of the variouscurves 24, 25, 26

and 27. Thus, other shapes of ionization distribution canbe obtained, .but the illustrated one having the maximum amplitude nearthe material surface will be preferred in most applications' In many instances the sample of material will also be irradiated from' the opposite side .by a similar beam generator, or the material may be turned over, to provide for more uniform interior irradiation.

The voltage V can be produced electronically by, for example, a wave shaping or generatingcircuit energized in phase with ,V to produce .Vg. IAlso V can be produced by the mechanical arrangement shown in FIG.

6' in which a synchronous motor 28 operates in synchronisrn with voltage V,,. Motor -28 has ashaft 29 towhich a magnetic recording drum 3it isattaehed, Around approximately half of the periphe'ry'of drum 3%) a magnetic signal is impressed having a magnitude corresponding to the'rnagnitudeof the voltage V such that when drum as is'rotated past a magnetic. transducer 31,. this transducer-31 produces a signal corresponding to-thewave Vg "during the positive half cycleof voltage Y fie. when w. electrons are emitted by filament, 8. 7 During thenegative half cycles it need not produce a signalflThesignal from transducer 31 is then amplifiedby an amplifier'32 and appliedby leads 33 acrossterminal-1t); thusSupplying the bias voltage for grid electIode'Q." .It' is noted that generator 29 and amplifierfil operate ata highvolta'ge level 'When electron beam Z isdeflected electrostatic or .electromagnetic ;defiection means (not shown), the rate .of such deflection is desirably highin comparison'to the frequency of V5; the accelerationivoltage.

It will be apparent to those skilled inthe art that in the case of beamscanning it .is necessary, of course, to synchronize thedefiection voltage and its magnitude with *thebeam voltage, V so'that deflection will be appropriate for the electron beamsenergy.

J In summary, an electronbeam generator and. a method of operation of this'beamigenerator'has beendescribed for producing an ionization distribution within thethickness' of an irradiated material, sothat the ionization near the surface of this material is comparable with the maximum ionization below the surface.1 .IThis isfin contrast 0 to a con'stant potential electron beam' -generator which produces an ionization distribution that is substantially v I less near the surface of the irradiated rnaterial than it is The improved ionization fdistribution near the surface is obtained by ivarying the beam currentin phase with thebeam voltage such that thecurrent is sufficiently low, at the low beam voltages, that destructive Window heating is avoided. In this way the most ionization that is com- I patible with non-destructive windowfheating is obtained at't-he low'beam' voltages, which are the voltages that prol ducei ienization near the surface of the irradiated-material. A sinusoidally shaped .half-lvva've grid voltage is herein iilustrated .as occurring .inphase with a similarqacceleratron voltage; It is understood these in phase signals may assume other waveformswithoutdepartingifrom the presintend, therefore, by the appended claims, to cover all 1 such modifications and changes as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an electron beam generator system for irradiating materials, an electron gun for producing an electron beam and including a grid electrode, an electron transmissive window separating said gun and said materials, means for applying a bias voltage to said grid electrode for producing space charge limited operation of said electron gun, means for accelerating said electron beam with a voltage whose magnitude varies as a function of time during substantially its entire period of operation, and means for applying a variable voltage to said grid electrode for causing the magnitude of the beam current to vary during substantially the same period and in proportion to the magnitude of said acceleration voltage.

2. In an electron beam generator system forirradiating material, an electron gun for producing an electron beam and including a grid electrode, a vacuum tight Window between said electron gun and said material through which said electron beam passes, means for producing space charge limited operation of said electron gun, means for accelerating said electron beam with a voltage having substantially a sine wave shape, and means for-applying a variable voltage to said grid electrode that causes the magnitude of the beam current to vary in accordance with a similar sine wave shape in phase with said voltage wave shape.

3. A method of operating an electron beam generator, having a grid electrode, to improve the uniformity of ionization within material that is impinged by an electron beam from the electron beam generator, comprising gradually varying the energy of said electron beam over a period of time, biasing the grid electrode to produce space charge limited operaiton of the electron beam generator, and applying a voltage to the gird electrode that causes the beam current to vary in magnitude substantially in proportion to the variations in the magnitude of the beam energy.

4. Apparatus for irradiating material comprising means for generating an electron beam directed tointercept said material, means for cyclically varying the energy of said electron beam, and means for simultaneously cyclically varying the electron beam current in phase with the cyclically varying energy to correspond in relative magnitude to the magnitude of said beam energy from near zero energy to near maximum energy.

5. Apparatus for irradiating material comprising means for generating an electron beam whose energy gradually assumes a plurality of diiferent energy level values at different times during a given operating cycle, means for directing said beam at a material to be irradiated, and means for varying the beam current magnitude as the beam energy varies and in substantial proportion thereto, to produce selectably increased irradiation at the surface of said material relative to the interior thereof.

6. The method of irradiating material comprising generating an electron beam whose energy assumes a plurality of different energy level values at different times during an interval, directing said beam at a material to be irradiated, and varying the total radiation of said beam in relation to the energy thereof so that at said different times the current of said beam assumes values generally proportional to its energy, whereby the surface of the said material receives an amount of radiation comparable to the interior of said material.

7. The method of irradiating material comprising generating an electron beam, directing the beam at a material to be irradiated, gradually raising the energy of said beam from a lower value to a higher value and concurrently gradually raising the current represented by said beam from a lower value to a higher value.

8. In an electron beam generator system for irradiating material, an electron gun for producing an electron beam and including a grid electrode, an electron permeable window through which said electron beam is directed toward the material to be irradiated, means for accelerating said electron beam with a portion of an alternating voltage wave which varies as a function of time, and means for applying a variable bias voltage to said grid electrode which voltage is proportional to said alternating voltage wave and in phase therewith to cause the magnitude ofthe beam current to vary as said acceleration varies so that low values of beam acceleration will not subject said window to damaging radiation.

References Cited by the Examiner UNITED STATES PATENTS 3,066,238 11/62 Arndt 250-495 FOREIGN PATENTS 874,195 7/59 Great Britain.

RALPH G. NILSON, Primary Examiner.

ARTHUR GAUSS, Examiner. 

1. IN AN ELECTRON BEAM GENERATOR SYSTEM FOR IRRADIATING MATERIALS, AN ELECTRON GUN FOR PRODUCING AN ELECTRON BEAM AND INCLUDING A GRID ELECTRODE, AN ELECTRON TRANSMISSIVE WINDOW SEPARATING SAID GUN AND SAID MATERIALS, MEANS FOR APPLYING A BIAS VOLTAGE TO SAID GRID ELECTRODE FOR PRODUCING SPACE CHARGE LIMITED OPERATION OF SAID ELECTRON GUN, MEANS FOR ACCELERATING SAID ELECTRON BEAM WITH A VOLTAGE WHOSE MAGNITUDE VARIES AS A FUNCTION OF TIME DURING SUBSTANTIALLY ITS ENTIRE PERIOD OF OPERATION, AND MEANS FOR APPLYING A VARIABLE VOLTAGE TO SAID GRID ELECTRODE FOR CAUSING THE MAGNITUDE OF THE BEAM CURRENT TO VARY DURING SUBSTANTIALLY THE SAME PERIOD AND IN PROPORTION TO THE MAGNITUDE OF SAID ACCELERATION VOLTAGE. 